Inkjet recording is advantageous in that it facilitates full-color printing because the process is easier and simpler than any other recording methods and it can provide high resolution images. As an inkjet ink, a dye-based ink is typically used in which various water-soluble dyes are dissolved in water or a mixed liquid containing water and an organic solvent. However, dye-based inks have a disadvantage in that they are poor in light resistance, although they are excellent in color tone definition. In the meanwhile, pigment-based inks in which carbon black or various organic pigments are dispersed are now actively studied because of their excellence in light resistance, as compared to dye-based inks.
However, pigment-based inks tend to cause nozzle clogging as compared to dye-based inks. Pigment-based inks are generally prepared by first preparing a dispersion liquid in which a coloring material and a dispersant are preliminary dispersed in an aqueous solvent such as water and alcohols, next dispersing the dispersion liquid to a certain degree using a medium-type dispersing device such as sand mill, and then diluting the dispersion to a specific concentration.
Typically, in a water-based pigment ink, a hydrophobic pigment is dispersed, and thus it is usual to use a dispersant such as surfactant and water-soluble resin. However, there is a problem that reliability of quality of obtained images with the use of such a pigment water-based ink is extremely poor. Then, in order to improve image quality, a technique is disclosed in which film-formable resin fine particles are added into an ink liquid. However, it is difficult to finely and stably disperse a plurality of components in a liquid for a long period of time. When a large amount of a dispersant such as surfactant is used to stably disperse fine particles, there are problems with occurrence of air bubbles in an ink tank or an ink head, degradation in image quality and the like. To improve dispersibility of these components, there have been a method of changing the surface of a pigment to a surface containing a hydrophilic group and a method of using a resin containing a hydrophilic group, and the like. However, employment of such a method causes a problem that when different type components are mixed, the components are unstably dispersed and the storage stability of the ink is degraded, although when one of these components is singularly used, it can be stably dispersed in a water-based ink liquid.
Patent Literature 1 discloses an ink that contains, together with a pigment, a polyether polyurethane resin having acid groups, for the purpose of improving glossiness of images and stability of the initial dispersion. Patent Literature 2 discloses an ink using, as a polyurethane resin, polycaprolactone polyurethane resin derived from polycaprolactondiol. Patent Literature 3 discloses an ink using polyether polyurethane resin derived from polyetherdiol. Patent Literature 4 discloses an ink using polyester polyurethane resin derived from an aliphatic polyesterdiol.
In addition, the following methods are also reported: a method in which a water-dispersible resin having a carboxyl group and a nonionic hydrophilic group in a molecule is dispersed in water (see Patent Literature 5); a method in which the polarity of a water-soluble polymer is identical to that of a surfactant or a nonionic surfactant is added in a water-based liquid (see Patent Literature 6); a method in which in an aqueous recording liquid, the polarity of a color ionic group-containing polyester resin is identical to that of hydrophilic groups of colorants (see Patent Literature 7); and a method in which the polarity of pigments is identical to that of a resin fine particle (see Patent Literature 8).
In addition, an aqueous inkjet ink composition is proposed which contains (i) a pigment dispersion liquid containing pigment particles having a specific particle size distribution in which at least 70% of the particles in the dispersion liquid have a diameter less than 0.1 μm and other particles in the dispersion liquid have a diameter of 0.1 μm or less, (ii) an aldehyde naphthalenesulfonate dispersant, and/or (iii) at least one type of sulfonic solvent (Patent Literature 9).
Also, Patent Literature 10 proposes a recording liquid which contains a pigment, a polymer dispersant and a nonionic surfactant. Patent Literature 11 and 12 propose using an AB block copolymer or a BAB block copolymer as a dispersant for pigments to be used. Further, Patent Literature 13 proposes using a specific pigment, a water-soluble resin and a solvent.
Although the incorporation of a resin emulsion into a water-based pigment ink to improve ink fixability has already been proposed, Patent Literature 14, for example, discloses an ink containing a wax emulsion, a resin emulsion, a latex, and an inorganic or organic super fine particle and having the minimum film forming temperature of 40° C. or higher. This document also discloses that examples of practicable resin emulsion include, solely or as a copolymer, an emulsion of vinyl acetate, acryl, styrene, olefin, and the like, the emulsion having the minimum film forming temperature of 40° C. or higher. In other words, there is no specific limitation to the resin composition, provided that only the condition of the minimum film forming temperature is met. This document is silent on advantages or disadvantages in the effects depending on the types of the resins, and the inks used in the examples show the same excellent performances and effects, irrespective of the resin types. However, since the resin emulsions shown in the examples use pigments having a large particle diameter of 1.0 μm or less, the resin emulsion does not fulfill the requirements of abrasion resistance, ink storage stability, and image density when a particle diameter of the pigment is 70 nm to 180 nm.
The present inventors have previously reported in Patent Literature 15 that by selection of a specific polyurethane resin (i.e., an anionic self-emulsifying polyurethane resin similar to the polyurethane resin used in the present invention), the requirements of image fixability, ink storage stability, and ejection stability are satisfied even when the particle diameter of the color pigment is in the range of 70 nm to 180 nm, and also recognized that furnace black and channel black are equally usable as a black pigment ([0028]). However, the present inventors' extensive studies performed thereafter reveal that in some cases a polyurethane resin can be aggregated by an organic solvent contained in the inkjet recording ink, resulting in impairment of image fixability, ink storage stability, and ejection stability. Accordingly, a resin emulsion satisfying all of the above performances simultaneously still remains to be discovered.
In the meantime, methods of dispersing a pigment without using a dispersant have been disclosed. Examples thereof include a method in which a substituent having a water-solubilizing group is introduced to a carbon black (Patent Literature 16); a method in which a water-soluble or the like is polymerized onto the surface or a carbon black (Patent Literature 17); a method in which a carbon black is oxidized (Patent Literature 18); and a method in which an oxidized carbon black and a ternary polymer having acrylic acid, styrene, and α-methylstyrene are incorporated into an ink to ensure water resistance and ejection stability (Patent Literature 19).
Additionally, an inkjet recording liquid is proposed which is characterized in that dispersed particles have a volume average particle diameter of 30 nm to 200 nm (Patent Literature 20).
However, none of the above-described conventional ink liquids can provide a sufficient, satisfiable image density with respect to black pigment ink, although they can exhibit high image density with respect to color pigment ink.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-290044
Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 09-031360
Patent Literature 3: Japanese Patent Application Laid-Open (JP-A) No. 2000-001639
Patent Literature 4: Japanese Patent Application Laid-Open (JP-A) No. 2002-167536
Patent Literature 5: Japanese Patent Application Laid-Open (JP-A) No. 05-239392
Patent Literature 6: Japanese Patent Application Laid-Open (JP-A) No. 08-283633
Patent Literature 7: Japanese Patent Application Laid-Open (JP-A) No. 2000-063727
Patent Literature 8: Japanese Patent Application Laid-Open (JP-A) No. 2001-081366
Patent Literature 9: Japanese Patent Application Laid-Open (JP-A) No. 08-333531
Patent Literature 10: Japanese Patent Application Laid-Open (JP-A) No. 56-147871
Patent Literature 11: U.S. Pat. No. 5,085,698
Patent Literature 12: U.S. Pat. No. 5,221,334
Patent Literature 13: U.S. Pat. No. 5,172,133
Patent Literature 14: Japanese Patent (JP-B) No. 2,867,491
Patent Literature 15: Japanese Patent Application Laid-Open (JP-A) No. 2004-169008
Patent Literature 16: U.S. Pat. No. 5,571,311
Patent Literature 17: Japanese Patent Application Laid-Open (JP-A) No. 08-081646
Patent Literature 18: Japanese Patent Application Laid-Open (JP-A) No. 08-003498
Patent Literature 19: Japanese Patent Application Laid-Open (JP-A) No. 09-194775
Patent Literature 20: Japanese Patent Application Laid-Open (JP-A) No. 2000-144028